1. Field of the Invention
The current invention relates to devices made from narrow-gap (and more conventional) semiconductors for operation at infrared wavelengths.
2. Discussion of Prior Art
Many prior art devices include a diode junction, hence metal contacts need to be formed to a region of n-type and a region of p-type material. Ideally the metal contact should be ohmic and have a low resistance, i.e. very much less than that of the junction. The surfaces of the semiconductors tend to comprise a native oxide, which is normally insulating to some extent and must be removed or disrupted to permit contact to the semiconductor underneath. The nature of the band-bending which often occurs at the metal-semiconductor interface means that it is relatively easy to obtain the desired contact to n-type material, but it can be difficult to achieve an ohmic, low resistance contact to the p-type semiconductor.
Alloying using a metal with a small proportion of a like-type dopant can be used with some semiconductors. When annealed, this gives a heavily doped region to which the metal is more likely to form an ohmic contact, for example Au/Zn for a p+ p contact to GaAs. This does not work well, however, with the narrow-gap semiconductors, such as indium antimonide (InSb) and cadmium mercury telluride (CMT) because of their low melting temperatures and the high diffusion rates of some of the dopants. Ion milling is often used for the narrow-gap materials to remove the surface oxide. This tends to result in near surface (typically tens of nanometers) damage which is heavily n-type in nature plus additional damage which can propagate significantly further into the material and degrade the junction properties. As a consequence the thickness of the contact regions in diodes made from the narrow-gap semiconductors needs to be greater than, or of the order of, 10xe2x88x926m.
In devices such as light emitting diodes and diode lasers where optical radiation passes through one or both contact regions, this thickness of material can lead to significant optical losses which will degrade the device performance, in some cases catastrophically.
Degenerate doping of the material, i.e. doping to a sufficiently high level that the Fermi energy lies within the appropriate band, leads to a Moss-Burstein shift whereby the absorption coefficient for radiation of energy just greater than the band-gap decreases so that it becomes transparent. Owing to the band structure of the narrow-gap semiconductors, however, it is extremely difficult to induce degenerate doping levels in p-type material, so that a sufficient Moss-Burstein shift is not observable. In contrast, it is comparatively easy to produce degenerate doping in n-type material and large Moss-Burstein shifts can be achieved. Hence the n-type contact regions of diodes can be made transparent to radiation generated or absorbed in the active region but the p-type contact region can not.
Also relevant to the present invention is U.S. Pat. No. 5,338,944. This document describes a light emitting diode for emitting visible radiation in the blue region of the visible spectrum, comprising an n-type silicon carbide substrate, an n-type silicon carbide top layer and a light emitting p-n junction structure between the n-type substrate and the n-type top layer. The device also includes, between the n-type top layer and the n-type substrate, means for coupling the n-type top layer to the light emitting p-n junction structure while preventing n-p-n behaviour between the n-type top layer, the p-type layer in the junction structure and the n-type substrate. This is achieved by means of a degenerate junction structure comprising a p-type portion and an n-type portion of silicon carbide. The p-type portion and the n-type portion are very thin, of the order of 250-1000 angstroms, with a very high doping concentration of at least 1xc3x9710xe2x88x9219 cmxe2x88x923. This invention relates to light emitting diodes operating in the blue region of the visible wavelength spectrum. It therefore relates to devices formed from material having a bandgap of at least 2.6 eV, such as silicon carbide.
Applied Physics Letts, 62(1993) 17 May, No. 20, New York, USA (A. R. Sugg et al.) relates to n-p-(pxe2x88x92-nxe2x88x92)-n AlyGa1-y-As-GaAs-InxGa1-xAs quantum-well laser with +-n+ GaAs-InGaAs tuned contact on n-GaAs. Hence, this document relates to laser devices formed on GaAs substrates.
According to the current invention, a semiconductor device comprises an active layer of p-type or n-type material forming a junction with a first layer of doped n-type material, a second layer of doped n-type material adjacent to a layer of doped p-type material, which may be adjacent to, or separated from by other layers, the active layer of p-type or n-type material, and means for providing electrical contact with the device including means for providing electrical contact, via the second layer of doped n-type material, with the adjacent layer of doped p-type material.
characterised in that the second layer of doped n-type material has a doping concentration of between 1xc3x971018 cmxe2x88x923 and less than 1xc3x971019 cmxe2x88x923 and in that semiconductor energy band-gap of the active layer is less than 0.5 eV.
The invention should not be regarded as being restricted to a particular set of doping levels.
Preferably one or both of the first and second layers of doped n-type material is transparent to radiation, of energy greater than that of the band-gap, which is emitted or absorbed by the device.
In a specific embodiment the device comprises a front surface emitting positive LED, negative LED or detector.
In another specific embodiment the device comprises a back surface emitting positive LED, negative LED or detector wherein the means for providing electrical contact, via the second layer of doped n-type material, with the adjacent layer of doped p-type material comprises a metallic contact and the second layer of doped n-type material provides a transparent front contact region to facilitate the use of the metallic contact as a mirror.
In another specific embodiment the device comprises a laser diode and the first and second layers of doped n-type material provide optical confinement within the active layer.
In another specific embodiment the second layer of doped n-type material provides an electrical path to an excluding or extracting contact. In this embodiment the device may comprise a field effect transistor or a bipolar transistor.
In another embodiment of the invention, the device may be a front surface emitting negative LED wherein the doping concentration of the doped n-type layer extends beyond 1xc3x971019 cmxe2x88x923.
In another embodiment of the invention, the device may be a back surface emitting negative LED, wherein the doping concentration of the doped n-type layer extends beyond 1xc3x971019 cmxe2x88x923, and wherein the means for providing electrical contact, via the second layer of doped n-type material, with the adjacent layer of doped p-type material comprises a metallic contact, and the second layer of doped n-type material provides a transparent front contact region to facilitate the use of the metallic contact as a mirror.